Cosmos greenstone terrane: Insights into an Archaean volcanic arc, associated with komatiite-hosted nickel sulphide mineralisation, from U-Pb dating, volcanic stratigraphy and geochemistry
De Joux, Alexandra
Joux, Alexandra de
The Neoarchaean Agnew-Wiluna greenstone belt (AWB) of the Kalgoorlie Terrane, within the Eastern Goldfields Superterrane (EGS) of the Yilgarn Craton, Western Australia, contains several world-class, komatiite-hosted, nickel-sulphide ore bodies. These are commonly associated with felsic volcanic successions, many of which are considered to have a tonalite-trondhjemite-dacite (TTD) affinity. The Cosmos greenstone sequence lies on the western edge of the AWB and this previously unstudied mineralised volcanic succession contrasts markedly in age, geochemistry, emplacement mechanisms and probable tectonic setting to that of the majority of the AWB and wider EGS. Detailed subsurface mapping has shown that the footwall to the Cosmos mineralised ultramafic sequence consists of an intricate succession of both fragmental and coherent extrusive lithologies, ranging from basaltic andesites through to rhyolites, plus later-formed felsic and basaltic intrusions. The occurrence of thick sequences of amygdaloidal intermediate lavas intercalated with extensive sequences of dacite lapilli tuff, coupled with the absence of marine sediments or hydrovolcanic products, indicates the succession was formed in a subaerial environment. Chemical composition of the non-ultramafic lithologies is typified by a high-K calc-alkaline to shoshonite signature, indicative of formation in a volcanic arc setting. Assimilation-fractional crystallisation modelling has shown that at least two compositionally distinct sources must be invoked to explain the observed basaltic andesite to rhyolite magma suite. High resolution U-Pb dating of several units within the succession underpins stratigraphic relationships established in the field and indicates that the emplacement of the Cosmos succession took place between ~2736 Ma and ~2653 Ma, making it significantly older and longer-lived than most other greenstone successions within the Kalgoorlie Terrane. Extrusive periodic volcanism spanned ~50 Myrs with three cycles of bimodal intermediate/felsic and ultramafic volcanism occurring between ~2736 Ma and ~2685 Ma. Periodic intrusive activity, related to the local granite plutonism, lasted for a further ~32 Myrs or until ~2653 Ma. The Cosmos succession either represents a separate, older terrane in its own right or it has an autochthonous relationship with the AWB but volcanism initiated much earlier in this region than currently considered. Dating of the Cosmos succession has demonstrated that high-resolution geochronology within individual greenstone successions can be achieved and provides more robust platforms for interpreting the evolution of ancient mineralised volcanic successions. The geochemical affinity of the Cosmos succession indicates a subduction zone was operating in the Kalgoorlie Terrane by ~2736 Ma, much earlier than considered in current regional geodynamic models. The Cosmos volcanic succession provides further evidence that plate tectonics was in operation during the Neoarchaean, contrary to some recently proposed tectonic models.